The present invention relates to a method and apparatus for projecting a photomask pattern onto and thereby exposing re-exposing substrates and/or semiconductor wafers, and, more particularly, to a method and apparatus for transcribing a standard photomask pattern of a master mask onto a plurality of re-exposing substrates, wherein the re-exposing substrates are used as work masks for patterning semiconductor devices, such as semiconductor chips, or onto a semiconductor wafer used for producing semiconductor devices.
There are three basic classifications of such methods for projecting a photomask pattern of a master mask onto, and thereby exposing, a re-exposing substrate: a contact method performed by directly attaching the re-exposing substrate to the master mask, a proximity method performed by bringing the re-exposing substrate proximately close to the master mask, and a projection method performed by projecting the image of the photomask pattern of a master mask onto a re-exposing substrate using an exposing optical system. The contact method and the proximity method have the merit that each method can be simply performed; that is, the contact method can be performed by simply directly contacting the re-exposing substrate with the master mask, and the proximity method can be performed by simply setting the re-exposing substrate in very close proximity to the master mask. However, these methods have the demerits, respectively, that a standard photomask pattern provided on the surface of the master mask is easily damaged, because the re-exposing substrate directly contacts or may readily contact the standard photomask pattern during the projecting and/or exposing process. Accordingly, recently, the projection method has become popular because the method can be performed without any contact between the surface of the re-exposing substrate and the master mask. By applying the projection method to the patterning process for the work mask production, the working life of the master mask can be increased and the production cost of the work mask can be reduced.
However, the projection method presents the problem that the accuracy of transcribing the photomask pattern onto the re-exposing substrate decreases. During projecting of the photomask pattern for exposing a plurality of the re-exposing substrates in the production of the work masks, it cannot be avoided that phenomena such as heat expansion, heat contraction, and mechanical vibration occur in the mask aligner. When these phenomena occur in the master mask, the re-exposing substrate, and/or the projecting optical system, the transcribing accuracy decreases, so that the photomask pattern transcribed on each work mask cannot be used because the photomask pattern is distorted.
Since the re-exposing substrates, after development, become the work masks and each work mask is applied to the production of semiconductor devices such as semiconductor chips, the quality of the photomask pattern, called the work photomask pattern, of each work mask greatly affects the cost of the semiconductor chips. For example, if the work photomask pattern has a defect and is applied to the production of the semiconductor chips without noticing the defect, the resulting semiconductor chip products are failures. Therefore, each work photomask pattern must be individually inspected even though the inspection takes a lot of time. Accordingly, it has been desired to shorten the time for the inspection, in other words, it has been desired to improve the prior art projecting and exposing method and apparatus so that the inspection can be easily made during the projecting and exposing process.
FIG. 1 is a schematic, cross-sectional view of a projecting and exposing apparatus of the prior art. The apparatus is made by Canon Corporation and named as "Mirror Projection Mask Aligner MPA 500 FA or 600 FA" which will be abbreviated as "mask aligner" hereinafter.
As shown in FIG. 1, the mask aligner comprises a light source unit 11 and a projecting optical unit 12 in which a carriage 32 is installed. The light source unit 11 is tightly mounted on a frame 41 of the projecting optical unit 12, and a carriage 32 is placed inside of the frame 41 so as to be movable relative to frame 41 in a direction M.
In light source unit 11, the light emitted from a light source 14, which is a xenon-mercury (Xe-Hg) lamp, is gathered by spherical mirrors 15 and 16 and reflected by a cold mirror 19, which ejects heat radiation emitted from light source 14, and passes through a slit 20 having the shape of a slender circular arc. The light which has passed through slit 20 is reflected by aluminum plane mirrors 21, 22 and 23 and spherical mirrors 17 and 18, and is led to projecting optical unit 12 after passing through a filtering shutter 24. The light which has arrived at the projecting optical unit 12 passes through a half mirror 25 and forms a light beam L having a cut shape, similar to the shape of slit 20, and which produces an image of slit 20 at the beam entrance of carriage 32. At the beam entrance of carriage 32, there is a mask holder 33 on which a master mask 101 is mounted. The light beam L which has passed through a standard photomask pattern of the master mask 101 is nearly perpendicularly reflected by one face of a trapezoidal reflector 26, condensed by a condensing mirror system 35 comprising a concave mirror 27 and a convex mirror 28, again reflected by the other face of trapezoidal reflector 26, and produces an image of the standard photomask pattern at the beam exit of carriage 32. At the beam exit of carriage 32, there is a mounting base 34 on which a re-exposing substrate 5 is mounted, onto which the image of the standard photomask pattern is projected. In other words, the mounted positions of master mask 101 and re-exposing substrate 5 are conjugate positions of the condensing mirror system 35.
The condensing mirror system 35 and trapezoidal reflector 26 are fixed to frame 41, which means that light beam L constantly passes along a fixed path, defined by the optical axis of the condensing mirror system 35, in frame 41, whereas carriage 32 is installed in frame 41 through an air bearing so that carriage 32 can move in direction M carrying master mask 101 and re-exposing substrate 5, the air bearing not being shown in FIG. 1.
Therefore, when the slender circular arc spotlight image of slit 20 is formed on the surface of master mask 101 and runs across the surface of master mask 101 perpendicularly to direction M, the carriage 32 is moved by an amount equal to the width scanned by the slender circular arc spotlight over the surface of master mask 101. Accordingly, by the movement of carriage 32 in direction M, the image of the standard photomask pattern of master mask 101 can be projected onto and thereby expose the re-exposing substrate 5.
The projecting and exposing process is generally performed by two steps using the mask aligner: an alignment step and an exposing step. In the alignment step, the mounted positions of master mask 101 and re-exposing substrate 5 are aligned so that the mounted positions coincide with the conjugate points of the condensing mirror system 35, and the surfaces of master mask 101 and re-exposing substrate 5 are positioned perpendicular to light beam L and the optical axis of the condensing mirror system 35, respectively. In the exposing step, the standard photomask pattern of master mask 101 is projected onto and thereby exposes the re-exposing substrate 5. In the production of the work masks, the alignment step is usually performed only once, and after the alignment step, the exposing step is performed on the rest of the re-exposing substrates, one by one, for saving production time.
The alignment step is carried out by: (1) previously providing a plurality of first positioning markers, marked on the surface of master mask 101 around the standard photomask pattern; (2) mounting master mask 101 on mask holder 33; (3) newly preparing a standard marker substrate on which a plurality of second positioning markers are provided in the same arrangement and pitch as the arrangement and pitch of the first positioning markers; (4) initially mounting the standard marker substrate, which is not shown in FIG. 1, on mounting base 34; (5) observing the marker images of the first and the second positioning markers using an alignment scope 31 shown in FIG. 1, where the image of the first positioning markers is obtained by light reflected from the first positioning markers and passed through a half mirror 25, a field lens 29, and reflectors 30a and 30b, and the marker image of the second positioning markers is obtained by light reflected from the second positioning markers and passed through trapezoidal reflector 26, convex mirror 28, concave mirror 27, half mirror 25, field lens 29, and reflectors 30a and 30b, and where the first and the second positioning markers are provided so that the respective marker images can be observed, superimposed on each other, by alignment scope 31; (6) detecting the deviation appearing between the first and the second marker images, paying attention also to whether there is improper orthongonality and runout, i.e., the above-noted optical distortions resulting from thermal expansion and contraction of parts, occurring between the marker images; and (7) aligning the position of carriage 32 by adjusting an air bearing controller, which is not shown in FIG. 1, until the deviation is minimized, as determined by observation through the alignment scope 31.
After the alignment step, the exposing step is carried out by: (1) removing the standard marker substrate from mounting base 34; (2) mounting re-exposing substrate 5 on mounting base 34; (3) exposing the standard photomask pattern of master mask 101 onto the re-exposing substrate 5; and (4) repeating the above steps (2) and (3) for the rest of the re-exposing substrates until all of the re-exposing substrates are exposed.
However, even though the alignment step is correctly carried out, there is still a problem in the exposing step; that is, there is a possibility of the deviation which appears between the marker images increasing, because undesirable mechanical vibration and temperature change may occur in the mask aligner during the exposing step, which requires a lot of time for exposing the photomask patterns onto a plurality of the re-exposing substrates. If the alignment step were carried out for each successive re-exposing substrate, the above problem could be avoided; specifically, and by analogy to the alignment step involving items (1) through (7), after the step (8) of developing the re-exposing substrate, the further step (9) of confirming whether the alignment had been correctly performed or not could be achieved by inspecting the deviation which appears between the first and the second marker images which are printed on the work mask by the development. Realistically, this is impossible to do because this occupies the mask aligner for too much time. However, if the work masks were applied to the production of the semiconductor devices without any inspection, the yield rate of the production of the semiconductor devices would be decreased. Therefore, usually, the work photomask patterns of the work masks are individually inspected, for example by a pattern comparing inspecting method, before applying the work mask to the production of the semiconductor devices.
As mentioned above, in the prior art, there is the problem that the transcribing accuracy cannot be kept high in the production of the work masks without individually inspecting the work photomask patterns of the work masks, and the latter requires a lot of time and this is undesirable.